1. Field of the Invention
The present invention relates to the curing of coated substrates which deform at elevated temperatures, and more particularly, to methods and devices for ultraviolet radiation curing of such coated substrates at lower temperatures than previously utilized.
2. Information Disclosure Statement
UV curable inks, adhesive and coatings are finding increasing use in the printing industry. The UV lamps used to cure these inks produce considerable energy in the visible and infrared spectra. This can cause the substrate which is being printed upon to reach a high temperature even when moving at high speed. This temperature will deform thin substrates, such as plastic sheets, plastic foils or styrene cups. When the press is stopped, the UV light must be shuttered so it does not burn the substrate.
The present invention reduces the temperature during printing and eliminates the need for a shutter on the ultraviolet curing equipment when the substrate is stopped under the lamps. In the present invention device and method, a shield, e.g. a heat absorbing glass with an IR reflective coating applied to one side, is placed between the UV lamp and the coated substrate. The present invention shield transmits the needed UV energy.
Typically the UV energy in the 360 nm region from a 300 watt/inch Hg vapor lamp is 800 mw/cm.sup.2. The temperature of the substrate under the lamp would exceed 250.degree. C. (482.degree. F.), if not shuttered. Using the present invention, the UV energy will be reduced to about 600 mw/cm.sup.2, but the temperature will be below 100.degree. F. (If required, the power into the lamp may be increased to 350 watts/inch to restore the UV to 800 mw/cm.sup.2 with a temperature of about 110.degree. F.) At reduced temperatures, achieved by the present invention, even the thinnest substrate can be stopped under the ultraviolet lamp without the need to interpose a mechanical shutter between the lamp and substrate. When a shutter is used, it is usual to reduce the power in the lamp so the shutter does not distort. These problems are eliminated with the present invention devices and methods. The power can still be reduced when using the present invention shield and the coated substrates will be only a few degrees above ambient.
In the instance stated, the prior art systems have the problem that the power cannot be reduced below about 100 watts/inch or the lamp will go out and at 100 watts/inch, the temperature is still too hot for most substrates when stationary under the lamp, which is why the shutter is needed. If the lamp were turned off, it takes too long to cool down to restart, e.g., about five minutes. While a microwave excited lamp warms up in seconds, the microwave lamps also produce too much heat for curing these coated substrates.
Notwithstanding the above prior art, there are no teachings or suggestions that would render the present invention anticipated or obvious.